1. Field of the Invention
The present invention relates to a dielectric ceramic composition and a monolithic ceramic capacitor using the same.
2. Description of the Related Art
Examples of conventional dielectric materials having flat temperature dependant capacitance characteristics include BaTiO3xe2x80x94Nb2O5xe2x80x94MgOxe2x80x94MnO type materials disclosed in Japanese laid-open No. 8-151260, BaTiO3xe2x80x94Ta2O5xe2x80x94ZnO type materials disclosed in Japanese laid-open No. 5-109319, and the like. These materials are fired in an air atmosphere and exhibit a dielectric constant of 2000 or more.
Since Ni or an Ni alloy is used for an internal electrode, there are also reports of monolithic capacitor dielectric materials which are not changed to semiconductors even by firing at low oxygen partial pressure, and which have flat temperature dependant capacitance characteristics, for example, such as BaTiO3xe2x80x94(Mg, Zn, Sr, Ca) Oxe2x80x94B2O3xe2x80x94SiO2 type materials disclosed in Japanese Examined Patent Publication No. 61-14611, (Ba, M, L) (Ti, R)O3 type (wherein M=Mg or Zn; L=Ca or Sr; R=Sc, Y or a rare earth element) materials disclosed in Japanese laid-open No. 7-27297, and the like. These materials exhibit a dielectric constant of as high as 2000 or more.
BaTiO3xe2x80x94Re2O3 type dielectric materials containing a rare earth element (Re) include materials which are fired in an air atmosphere, as disclosed in Japanese Examined Patent Publication No. 63-10526.
A conventional monolithic ceramic capacitor using any one of these dielectric ceramic compositions is frequently used under a low-frequency-low-voltage alternating current, and under a low-voltage direct current.
In recent years, the operation conditions under which a monolithic ceramic capacitor operates have been increasingly made severe with advances in integration, function and cost reduction of electronic apparatus. Therefore, there is great demand for a reduction in loss, improvements in insulation, dielectric strength and reliability, and an increase in capacity of the monolithic ceramic capacitor. Cost reduction is also increasingly required.
While the dielectric materials disclosed in Japanese laid-open Nos. 8-151260 and 5-109319 and Japanese Examined Patent Publication No. 63-10527 achieve a high dielectric constant, they have the fault that use under a high-frequency-high-voltage alternating current causes a high loss and generates much heat. In firing in an atmosphere which allows the use of Ni or a Ni alloy for the internal electrodes in order to decrease the cost, the ceramics are changed to semiconductors, and thus a noble metal such as an expensive Pd, Agxe2x80x94Pd or the like must be used for the internal electrodes.
The dielectric materials disclosed in Japanese Examined Patent Publication No. 61-14611 and Japanese laid-open No. 7-272971 have dielectric constants of as high as 2000 or more and 3000 or more, respectively, and a low rate of change in capacitance with temperature, but use under high-frequency-high-voltage alternating current causes a high loss and generates much heat. These dielectric materials have anti-reducing characteristics and thus permit the formation of a monolithic ceramic capacitor comprising internal electrodes made of a base material such as Ni or the like, but the dielectric strength and reliability are low in use under a high-voltage direct current.
Particularly, there is an increasing demand for decreasing the loss and heat generation of a monolithic ceramic capacitor with advances in integration of an electronic apparatus. Monolithic ceramic capacitors have been increasingly used under a high-frequency-high-voltage alternating current, decreasing the life of the capacitors by the loss and heat generation of the monolithic ceramic capacitor. Also the loss and heat generation cause a temperature rise in a circuit, and thus causes an error in the operations of peripheral parts and a reduction in life. Because conventional dielectric ceramic compositions cause high losses and generates much heat particularly under a high-frequency-high-voltage alternating current, they cannot be used in a circuit under a high-frequency-high-voltage alternating current.
In addition, the monolithic ceramic capacitor is increasingly used under a high-voltage direct current. A monolithic ceramic capacitor comprising internal electrodes made of Ni has low resistance to direct-current voltage, and thus has a problem in that insulation, dielectric strength and reliability significantly deteriorate in use under a high-strength electric field.
The present invention provides a dielectric ceramic composition exhibiting a firing temperature of 1300xc2x0 C. or less, and a dielectric constant of 200 or more, and causing low loss and heat generation under a high-frequency-high-voltage alternating current, specifically a loss of 0.7% or less at 300 kHz and 100 Vp-p, a product (CR product) of insulation resistance and capacitance of as high as 7000 xcexa9xc2x7F or more at room temperature under a high-voltage direct current, specifically, at a high electric field strength of 10 kV/mm, temperature characteristics of capacitance which satisfy the B characteristics defined by JIS standards and the X7R characteristics defined by EIA standards, and excellent properties in a high-temperature load test.
The present invention also provides a monolithic ceramic capacitor comprising a dielectric layer made of the above-described dielectric ceramic composition, and internal electrodes which can be made of not only a noble metal such as Au, Pd or an Agxe2x80x94Pd alloy but also a base metal such as Ni or an Ni alloy.
According to one aspect of the present invention, a dielectric ceramic composition includes a main component comprising a barium titanate solid solution and additive components, and a sintering additive, wherein when the main component is represented by the formula, ABO3+aR+bM (wherein ABO3 represents the barium titanate solid solution having a perovskite structure; R represents an oxide of at least one metal element selected from La, Ce, Pr, Nd, Sm, Eu, Gd, Tb, Dy, Ho, Er, Tm, Yb and Lu; M represents an oxide of at least one metal element selected from Mn, Ni, Mg, Fe, Al, Cr and Zn; a and b respectively represent the molar ratios of the oxides in terms of a chemical formula containing one metal element), in which 0.950xe2x89xa6A/B (molar ratio)xe2x89xa61.050, 0.12 less than axe2x89xa60.30, and 0.04xe2x89xa6bxe2x89xa60.30.
The content of the sintering additive may be about 0.8 to 8.0 parts by weight based on 100 parts by weight of the main component.
The main component may further contain, as an additive component, at least one of X(Zr, Hf)O3 (wherein X is at least one metal element selected from Ba, Sr and Ca), and D (wherein D is an oxide of at least one metal element selected from V, Nb, Ta, Mo, W, Y and Sc).
The content of X(Zr, Hf)O3 may be 0.35 mole or less based on 1 mole of barium titanate solid solution represented by ABO3 in the main component.
The content of D representing an oxide may be about 0.02 mole or less in terms of a chemical formula containing one metal element based on 1 mole of barium titanate solid solution represented by ABO3 in the main component.
The barium titanate solid solution represented by ABO3 may be represented by {(Ba1xe2x88x92xxe2x88x92ySrxCay)O}mTiO2 (wherein 0xe2x89xa6x+yxe2x89xa60.20, and 0.950xe2x89xa6mxe2x89xa61.050).
The sintering additive may comprise an oxide containing at least one of B and Si.
The monolithic ceramic capacitor of the present invention comprises a plurality of dielectric ceramic layers, internal electrodes formed between the respective dielectric ceramic layers and external electrodes electrically connected to the internal electrodes. In the monolithic ceramic capacitor, the dielectric ceramic layers comprise the above-described dielectric ceramic composition.
The internal electrodes may comprise a sintered layer of a conductive metallic powder or a sintered layer of a conductive metallic powder containing glass frit.
The external electrodes may comprise a first layer comprising a sintered layer of a conductive metallic powder or a sintered layer of a conductive metallic powder containing glass frit, and a second layer comprising a deposited layer formed on the first layer.